In Colorado and other areas of the United States are located what are popularly known as "oil shales" occasionally exposed at the surface of the ground but generally overlaid by overburden to varying depths. Oil in the form of kerogen is entrapped within the shale deposits. For many years efforts have been made to recover the oil, and several processes have been proposed for the purpose. Many proposals have involved first the mining of the shale and then the surface extraction of the oil from the mined shale. The mining techniques and associated extraction techniques have generally involved intolerably high capital investments, energy expenditures, ecological damage, and extraction and refining costs.